Icon in custom:button-card disappears when nested under conditional card

Use a custom:button-card to flash a grid square when there’s unread e-mail in my inbox. When there’s no new e-mail, the icon should be invisible, thus, I use the conditional card to control that along with a binary_sensor based upon the state of the IMAP sensor. If the value is 0 unread e-mails, hide, otherwise, flash the e-mail icon.

Problem:

A custom button card fails to display the entity icon in a grid square when the grid card is under a conditional card.

Any ideas?